1. Field of the Invention
The present invention relates to a piezoelectric resonator used in, for example, an oscillator or a filter, and a method for manufacturing such a resonator. More specifically, the present invention relates to an energy-trap piezoelectric resonator operating in a thickness longitudinal vibration mode and a method for producing such a resonator.
2. Description of the Related Art
Conventionally, in order to form an oscillator or a piezoelectric filter, various types of piezoelectric resonators, such as a resonator using a fundamental wave in a thickness longitudinal vibration mode and a resonator using a higher mode, have been proposed. Piezoelectric resonators are required to have a high Qe value for a mode to be used, but a low Qe value for a spurious mode.
However, for, for example, a piezoelectric resonator using a third harmonic wave in a thickness longitudinal vibration mode, although methods for restricting a spurious wave in a mode that is of a lower order than a mode of, for example, a fundamental wave have been proposed, not many methods for restricting a higher-mode spurious wave have been proposed under present circumstances.
Japanese Unexamined Patent Application Publication No. 4-369914 proposes a piezoelectric ceramic resonator which can restrict a higher-mode spurious wave. Here, restriction of a dynamic range of resonance by a fifth harmonic wave having a frequency range that is greater than 20 MHz in a piezoelectric ceramic resonator having a frequency range of 12 MHz to 20 MHz and making use of resonance by a third harmonic wave in a thickness longitudinal vibration mode is proposed. More specifically, a piezoelectric ceramic material having an average crystal grain size of 3 xcexcm to 5 xcexcm, a maximum crystal grain size of 6 xcexcm at most, a maximum pore size of 8 xcexcm at most, and a porosity of 2% at most is used. By using this piezoelectric ceramic material, propagation of ultrasonic waves having a frequency that is greater than 20 MHz is restricted while ultrasonic waves having frequencies from 12 MHz to 20 MHz are properly propagated, so that the dynamic range of resonance by the fifth harmonic wave is restricted.
In the piezoelectric resonator disclosed in the aforementioned document, as described above, when a third harmonic wave in a thickness longitudinal vibration mode is used, the fifth harmonic spurious wave is restricted by restricting the dynamic range of the fifth harmonic wave in the thickness longitudinal vibration mode. However, as is clear from the above-described structure of this related technology, the document merely discloses that, when the frequency range of the third harmonic wave is from 12 MHz to 20 MHz and that of the fifth harmonic wave is greater than 20 MHz, it is effective to control the average crystal grain size, the maximum crystal grain size, the maximum pore size, and the porosity within the aforementioned respective particular ranges. In other words, the document does not disclose any method for restricting a spurious wave in a mode that is of a higher order than the mode that is used within various other frequency ranges. In addition, the document does not disclose any method for increasing the dynamic range of a resonant frequency that is used regardless of the frequency value.
In order to overcome the shortcomings and problems described above, preferred embodiments of the present invention provide a piezoelectric resonator which makes use of a thickness longitudinal vibration mode and which can, even if the piezoelectric resonator is formed of various piezoelectric materials and have various sizes, sufficiently increase the response to the thickness longitudinal vibration mode that is used, without limiting the frequency to a particular frequency, and also provide a method for manufacturing such a novel piezoelectric resonator.
Preferred embodiments of the present invention also provide a piezoelectric resonator which may be formed of various piezoelectric materials and may have various sizes, and which, not only has a sufficiently high response to a mode that is used, but also reliably restricts a higher-mode spurious wave.
In a first preferred embodiment of the present invention, a method for producing an energy trap piezoelectric resonator which operates in a thickness longitudinal vibration mode and which includes a piezoelectric member and a first vibrating electrode and a second vibrating electrode that are provided on respective major surfaces of the piezoelectric member and that overlap each other at a portion of the piezoelectric member includes the steps of forming the piezoelectric member using a piezoelectric material having an R value and an A value that are such that Qe=C/(Rxc3x97A), where R represents the average pore size in micrometers, A represents the porosity in percent, Qe is a value at a target frequency, and C is a constant that is uniquely determined by the piezoelectric material of the piezoelectric member, and forming the first and second vibrating electrodes onto the respective major surfaces of the piezoelectric member.
In one preferred embodiment of the present invention, when Qe (S) is to be substantially equal to or less than Qe (max) and Qe at the target frequency is to be substantially equal to or greater than Qe (min), the piezoelectric member is formed using a piezoelectric material having an R value and an A value that are such that C (S)/Qe (max)xe2x89xa6(Rxc3x97A)xe2x89xa6C/Qe (min), where Qe (S) is a Qe value at a frequency of a spurious wave in a mode that is higher than a mode of a wave at the target frequency, Qe (max) is an upper limit of Qe (S), Qe (min) is a lower limit of Qe, and C (S) is a constant that is uniquely determined by the piezoelectric material and satisfies Qe (S)=C (S)/(Rxc3x97A).
According to a second preferred embodiment of the present invention, an energy trap piezoelectric resonator which operates in a thickness longitudinal vibration mode includes a piezoelectric member made of a piezoelectric material having an R value and an A value that are such that Qe=C/(Rxc3x97A), where R and A represent the average pore size in micrometers and the porosity in percent of the piezoelectric member, respectively, Qe is a value at a target frequency, and C is a constant that is uniquely determined by the piezoelectric material of the piezoelectric member, and a first vibrating electrode and a second vibrating electrode that are provided on respective major surfaces of the piezoelectric member and that partially oppose each other with the piezoelectric member disposed therebetween.
In one preferred embodiment of the present invention, when Qe (S) is to be equal to or less than Qe (max) and Qe at the target frequency is to be equal to or greater than Qe (min), the piezoelectric member includes a piezoelectric material having an R value and an A value that are such that C (S)/Qe (max)xe2x89xa6(Rxc3x97A)xe2x89xa6C/Qe (min), where Qe (S) is a Qe value at a frequency of a spurious wave in a mode that is higher than a mode of a wave at the target frequency, Qe (max) is an upper limit of Qe (S), Qe (min) is a lower limit of Qe, and C (S) is a constant that is uniquely determined by the piezoelectric material and satisfies Qe (S)=C (S)/(Rxc3x97A).
Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.